I've used one and owned a couple of the basic Kephart spearpoint style. They are useful, but not my choice for an all around knife. The idea behind them was one knife for everything.....it does many things well, but nothing does everything perfect.
If going for one to do everything I prefer the design/style of Charlie May's Skifa with a scandi grind and use it for a lot of work...but I have a problem adhering to the one for everything principle.